This invention relates to an apparatus for seam welding, in which a metal plate or a surface-clad metal plate in a cylindrical form with a strip-like overlap section constituted by two overlapping edge portions is passed between pair roller electrodes via upper and lower wire electrodes for seam welding of the overlap section between the roller electrodes and also between the upper and lower wire electrodes under pressure applied to the section, and also which is provided with a welding current source for supplying a welding current between the roller electrodes.
It is well known in the art that a weld zone having excellent leakage-proof property can be obtained by seam welding, which is a well-known process of resistance welding. For this reason, seam welding is used for the welding of liquid transportation pipes, fuel tanks, drum cans and food cans. Among various seam welding process, lap seam welding is most usual. In the lap seam welding, a process material having a cylindrical form which is made from steel plate or the like, is fed such that an overlap section constituted by two overlapping edge portions of the process material is clamped between a pair of, i.e., upper and lower roller electrodes, each of which has a disk-like shape. As the material is fed with pressure applied to the overlap section, current is passed intermittently between the upper and lower roller electrodes to effect thermal fusion and welding by joule heat produced at this time. In this welding process, a large current can be passed in a short period of time to provide an increased welding speed. For this reason, the process is suited for mass production. In addition, unlike spot welding, nuggets are formed continuously during the current supply such that adjacent nuggets overlap each other, thus forming a continuous weld zone. Therefore, the same welding permits weld joints having excellent-air and water-tightness to be obtained and is thus said to be suitable for manufacturing food cans and drink cans.
However, the food cans and drink cans are required to provide corrosion resistance with respect to the contents, such as foods, drinks or the like, so tin-plated steel sheets can be used directly as process material of the food cans and drink cans, instead of cold-rolled steel sheets. When the tin-plated steel sheet material is subjected to seam welding, by using the upper and lower roller electrodes clamping the overlap section of the tin-plated sheet therebetween, there arises a transference of tin from the surface of the overlap section that is present on the surface to the outer periphery, i.e., contact surface, of the roller electrodes, which results in a tin contamination.
To solve this problem, there has been proposed and practiced an apparatus for lap seam welding using wire electrodes, as disclosed in Japanese Patent Publication 25,213/1969. In this lap seam welding apparatus, using wire electrodes, as shown in FIG. 1, upper and lower roller electrodes 1 and 2 are supported for rotation about respective axes 1a and 2a. Wire electrodes 3 such as copper wire runs along a portion of the periphery of each of the roller electrodes 1 and 2. An overlap section S of a process material 4 having cylindrical form is guided continuously together with the wire electrodes 3 between these wire electrodes 3, by means of which a seam welding of the overlap section is effected. In this case, the upper and lower roller electrodes 1 and 2 are rotated continuously. On the outer periphery of each of the upper and lower electrodes 1 and 2, there is formed a respective guide groove. The wire electrodes 3, which are fed along each of these guide grooves to between the upper and lower roller electrodes 1 and 2, runs together with the material 4 process material 4, by using the rotation of the upper and lower roller electrodes 1 and 2. Current is applied between the upper and lower roller electrodes 1 and 2 only while the material 4 is passed between the wire electrodes 3, whereby the overlap section S is thermally fused under pressure and welded.
With the prior art seam welding apparatus, however, it is difficult to detect the leading and trailing ends Sa and Sb of the overlap section S. Further, although it is necessary to vary the supplied welding current for the leading and trailing ends Sa and Sb and intermediate portion Sc other than the ends Sa and Sc, the adjustment is very difficult. Unless the adjustment is satisfactory, the welding performance is deteriorated.